This is an extremely rare case and has only happened twice to my knowledge on my server but it is there. Perhaps it has something to do with #130 but I'm not sure.

Sometimes when a player finishes a Quest, the money defined as a reward in the quests.yml is what their balance is actually set to and it doesn't give them that on top of what they had. Very weird glitch and no errors in the console whatsoever. Might need looking into.